Characterizing tissue species using nuclear magnetic resonance (“NMR”) can include identifying different properties of a resonant species (e.g., T1 spin-lattice relaxation, T2 spin-spin relaxation, proton density). Other properties like tissue types and super-position of attributes can also be identified using NMR signals. These properties and others may be identified simultaneously using magnetic resonance fingerprinting (“MRF”), which is described, as one example, by D. Ma, et al., in “Magnetic Resonance Fingerprinting,” Nature, 2013; 495(7440): 187-192.
Conventional magnetic resonance spectroscopy (“MRS”) offers a unique opportunity to noninvasively characterize metabolic profiles in a vast array of pathologies due to its chemical specificity. Phosphorus-31 (31P) MRS has been used extensively to assess the energetics of living tissues. In addition to measuring the concentrations of phosphate metabolites involved in tissue metabolism, 31P magnetization transfer (MT) techniques have been developed and successfully applied to the quantification of phosphocreatine (PCr) synthesis rate via creatine kinase in heart, skeletal muscle, and brain. The use of 31MT-MRS for measurement of ATP turnover has also been explored in vitro in cells and perfused organs, and in vivo in laboratory animals and humans. However, 31P MT-MRS has found limited utility due to its long data acquisition time needed to compensate for the low metabolite concentrations.
Strategies for shorter acquisition times have been pursued to partially ameliorate this problem. The first strategy uses a reduced number of spectra acquired under partially relaxed conditions, such as in the four angle saturation transfer (FAST) method, or the triple repetition time saturation transfer (TRiST) method. Reducing the number of acquired spectra is particularly useful in combination with CSI methods for spatially localized measurements. However, because the number of acquired spectra approaches the number of parameters to be estimated, the spectra must be of high SNR to achieve satisfactory robustness. Hence, these methods are limited for inherently low SNR applications.
The second strategy to shorten acquisition times reduces the number of unknown parameters by assuming an intrinsic T1 value for PCr, and includes techniques, such as the two repetition time saturation transfer (TwiST) and T1 Nominal methods. However, while several studies have reported no detectable changes in the T1 of PCr under pathological conditions, this does not guarantee that T1 of PCr remains constant in all pathologies and thus fixing the T1 value may lead to measurement errors. Because of this, 31P MT-MRS has only found limited utility in evaluating ATP synthesis, which is the most important step in cellular metabolism.
Thus, it would be desirable to provided additional systems and methods for MRS that overcome the aforementioned deficiencies.